1. Field of the Invention
The present invention relates to developing apparatuses used for image forming apparatuses such as copiers, printers, and facsimiles.
2. Description of the Related Art
Conventionally, electrophotographic image forming apparatuses optically scan an original image portion, which is an outer circumferential surface of a uniformly charged photosensitive drum, to form an electrostatic latent image and this electrostatic latent image is visualized using a toner, which is a colored resin. Such image forming apparatuses are capable of forming an image at a high speed and thus have been used widely in digitized printers and copiers, for example.
In recent years, there have been increasing demands particularly for forming images in color, and also in the electrophotographic image forming apparatuses, image forming apparatuses for forming a full color image consisting of toner images of four colors, cyan, magenta, yellow, and black, have been realized. In such image forming apparatuses, there has been recently an increasing demand for reducing the size of the body of the apparatuses in order to achieve space-saving, in addition to demands for increasing the speed and the image quality.
On the other hand, as a developer used for development of the electrostatic latent image, two-component developers constituted by a carrier and a toner, with which a high image quality and a low running cost can be realized, have been used widely. In this case, the electrostatic latent image that is formed on the surface of the drum is developed, using a developing apparatus provided with a development sleeve having magnets disposed therein, by rubbing the developer in the form of a magnetic brush against the surface of the photosensitive drum.
In such a developing apparatus, two screws are disposed horizontally in the developing apparatus, and homogenous mixing of the toner and the carrier and frictional charging of the toner can be performed sufficiently by circulating the developer between these two screws.
In this configuration, since the two screws are disposed in the horizontal direction, the developing apparatus was enlarged transversely when viewed from a cross section in a direction that is orthogonal to the shafts of the screws. Thus, there has been a problem in that when such a developing apparatus is used for a 4-drum tandem type image forming apparatus in which image forming units corresponding to various toner colors are disposed side by side in the transverse direction, the spacing between adjacent photosensitive drums is increased and thus the size of the body of the image forming apparatus is increased.
As a developing apparatus for solving this problem, for example, JP H10-31363A discloses a developing apparatus employing a vertically circulating system. In this developing apparatus, a development housing in which a development sleeve is accommodated is partitioned into an upper part and a lower part. The development sleeve is disposed in the lower part, and a screw is provided in each of the upper part and the lower part along the axial direction of the development sleeve, with the screws delivering the developer in opposite directions.
Furthermore, accumulation of the developer in a drawing-up portion and stress imposed on the developer, which are caused by drawing up the developer from the screw in the lower part to the screw in the upper part against gravity, are prevented by drawing up the developer using a magnet roller. In this way, the developing apparatus in which the developer is transported smoothly in the vertical direction and the width of the cross section of the apparatus is narrower in the direction that is orthogonal to the shafts of the screws is realized.
However, in the conventional developing apparatus employing a vertically circulating system as described above, the developer is transported by the screw provided in the upper part, immediately after the developer is drawn up. For this reason, the ability to mix the developer in the drawing-up portion is poor, so that it takes much time to render the developer uniform, the toner density of which has been made uneven through development.
Moreover, depending on the timing of toner replenishment, which is performed downstream of the drawing-up portion, the unevenness of the toner density of the developer may be promoted even more.
On the other hand, if the size of the developing apparatus is further reduced, the time for stirring the developer during each cycle of circulation is reduced, and furthermore, the capacity of the screw to stir the developer also is decreased with a reduction in the diameter of the screws. That is to say, a reduction in size leads to a problem in that the time needed for uniformly mixing the toner until the toner density is stabilized is increased, and uneven density is likely to occur in an output image.
Moreover, the developing apparatus employing a vertically circulating system as disclosed in the above-mentioned conventional art has a structure in which the developer is drawn up from the lower part into the upper part. For this reason, if the upper part into which the developer is drawn up does not have a sufficient space and sufficient transporting capacity for receiving the developer that has been transported by the screw in the lower part, accumulation and clogging of the developer occur in an end portion of the screw in the lower part.
When the accumulation of the developer is present in a portion on the screw that corresponds to an image printing area, there will be a problem in that the amount of the developer that is formed in a layer on the development sleeve is changed, resulting in an uneven image density.
Moreover, in the accumulation of the developer, great stress is imposed on the developer, which also will result in a problem of deterioration of the developer.
Furthermore, when clogging of the developer occurs, the screws may be broken and the apparatus may no longer function as a developing apparatus, in the worst case.
In particular, when further reductions in the size of the developing apparatus are to be achieved, the amount of the toner in the developing apparatus decreases in proportion to the size of the apparatus, and thus a difference in density between left and right portions of the output image is likely to occur when the toner is consumed through development. In order to prevent such a difference in density from occurring, it is necessary to increase the speed of circulation of the developer. In this case, it also is necessary to increase the speed of drawing-up, so that accumulation and clogging of the developer and deterioration of the developer become more likely to occur.